Mineral oxides have been used for centuries as pigments for both surface and integral coloring. Currently synthetic mineral oxides, ferric oxide, ferrous oxide, chromium green, cobalt blue, and titanium oxide, to name a few, are used as pigments for the coloring of hydrates. Mineral oxide pigments of different particle sizes can be used to color various water based materials, such as concrete; however, pigments having smaller particle sizes, less than 75 microns, are generally preferred. The pigment particles are of a variety of shapes and sizes with the smaller sized mineral oxide pigment particles (both natural and synthetic mineral oxides) having a greater total surface area percentage as compared to the total percentage of surface area in the larger sized pigment particles. This means that if two groups of pigment particles having the same mass/weight were compared side by side, with one group of pigment particles having an average particle size diameter that was equal to half the average diameter of the other group of pigment particles, the group with the smaller particle size diameter would have roughly twice the surface area. The greater surface area means that if everything else is equal, the smaller particle size pigments would have a much higher tinting strength than similar pigment particles of a coarser grind and larger size. The higher tinting strength allows for a lesser amount of pigment to be used, which presumably makes the smaller sized pigments more economical to use than coarsely ground pigment particles.
While mineral oxide pigments of a smaller particle size are advantageous because they have a higher tinting strength, they are disadvantageous because the handling characteristics are undesirable. Smaller sized pigment particles tend to build-up on mixing equipment used to produce the pigments, which means that the equipment must be extensively cleaned when the pigment colors are changed. Additionally, the smaller size pigment particles tend to remain on the walls of the packaging containing the pigment particles leading to the loss of some of the pigment product. Thus, the advantages gained by having to use lesser amounts of pigment particles, as a result of having a higher tinting strength, are diminished because some of the product remains in the packaging. Another problem is that as a class the smaller size pigment particles do not flow freely, meaning that even though the particles have a higher tinting strength they do not readily disperse in hydrates or similar materials. The lack of disperseability of the smaller pigment particles is in part attributable to the angle of repose which is equal to about 50.degree.. An increased angle of repose results in decreased flowability and disperseability of the pigments. Because the small pigment particles are not readily dispersible, this results in a longer mixing time when the pigments are added to the hydrates. Consequently, costs are increased because it takes more time and energy to mix the pigments with the hydrates, more time is required to clean the equipment, and not all the pigments are used.
A further problem associated with smaller sized pigment particles is that they tend to cake and form lumps when kept in storage for extended periods of time. This is especially disadvantageous because the lumps of the pigment particles may not completely disperse when mixed with hydrates or similar materials, resulting in a hydrate that is not uniformly colored. The lumped pigments may also cause the finished hydrate or cement product to have lumps of undispersed pigment in the material matrix meaning that the material may less structural strength.
Another problem associated with smaller sized pigment particles is the creation of dust. In a pigment production plant, dust is generated at nearly every step involved in the processing of the pigments, including the drying, grinding, transporting, and packaging steps. As a result, these production plants spend a great deal of time and money on housekeeping to prevent cross contamination of products. Besides increasing the costs of the production of the pigment particles, mineral oxide pigment dust can be a possible source of pollution leading to public relations problems.
Currently, known technologies include two options for making mineral oxide pigment particles which have a high tinting strength and which are dustless. The first option involves producing the mineral oxide pigment particles in an aqueous slurry, which can be advantageous because the pigment slurries are pumpable and are readily measurable by weight or volume. However, slurries suffer from the disadvantage that they are not very cost effective because the slurries weigh more than dry pigments and thus shipping costs increase as a result of including water with the pigments. Also, to achieve a slurry with a high solids content, a chemical additive must be included in the slurry thereby further increasing the cost of the pigments. Another problem associated with slurries is that they tend to settle and dry out if stored for extended periods of time or if not properly stored. One other problem associated with slurried pigments is that the metering equipment must be cleaned regularly to prevent cloggin and the equipment must be thoroughly cleaned when changing colors.
The second option for making pigments having a high tinting strength is for the pigment slurry to be spray dried to remove the water. Spray drying results in free flowing and dustless pigments agglomerated in bead form having an angle of repose of about 34.degree.. The pigment beads will readily redisperse in high slump concrete (ready mix and wet cast concrete); however, in zero or low slump concrete (block, paver and retaining wall stones) the redispersion can be slower than powdered pigments. Slow redispersion of the spray dried mineral oxide pigments is attributable to the lack of porosity of the spray dried pigments. The slow redispersion of the spray dried mineral oxide pigment beads is also a problem because higher production block plants for making blocks generally cannot devote the additional time required to get complete dispersion of the beaded pigments.
It is desirable to have a method for producing pigment particles that have high tinting strength and which are free of the problems mentioned above. Specifically, because of the problems associated with small sized mineral oxide pigment particles, it is desirable to have: a process for forming dry mineral oxide pigment particles that are non-dusting; pigment particles that readily disperse in all types of hydrates in a short amount of time; and, pigment particles that do not clump when stored for long periods of time, nor break down with the passage of time. As will be seen, the present inventive method and compositions eliminate the aforementioned problems.